I couldn't find answers on the Internet to some of my own questions, so I made some tests. I wanted to share my results. I hope some of them could be useful.
1) Enfeeble is applied to itself
Enfeeble extends the duration of hostile effects. And enfeeble is an hostile effect iself.
Enfeeble does extend its own duration. That means, for example, that Heart Seeker applies 30s of Enfeeble on non-resistant foes (without taking into account INT and PL).
Additional tip : that also applies to Weakening abilities targeting vulnerable foes, which makes Toxic Strike extremely potent against them (since its damages increase with duration).
2) Stoic Steel can charge up before battle
So if you start battle after 18s of immobility, you start with +3AR. I've never been aware of it before today.
This makes Stealth interesting for Paladins ^^
Additional tip : some of the "teleporting" abilities such as escape don't cancel Stoic Steel. I haven't test all of them.
3) Gambit refund only works on first hit/main target
Even if you use a multi-shot (classic blunderbuss) or AoE weapon (mortars), you'll get the refund only if 1st roll on main target.
So to get the complete refund, you'll need to dual wield and score 2 crits with the 2 "main" attacks. It is still good to get some AoE crits, but don't except a refund.
I haven't test it with WotEP.
4) Heart Seeker doesn't apply Bounce or AoE.
I have tested it with Watershaper focus, and only foes in the ability line can be hit. There is no double hits due to Bounce or rod's AoE.
5) Whirling Strikes does apply Bounce.
You need to equip a melee weapon to perform whirling strike. However, if you hold a ranged weapon in your other hand, you'll attack with it too, and it will even trigger Bounce with Driving Flight.
This makes Melee & Ranged dual wield somewhat attrative for ranged, especially with Scordeo's Trophy.
6) "Thunder Rolled..." upgrade
"...its crash could not be denied" apply a +15 Accuracy bonus, which is fine IMHO. The description does not specify it but you can read it by picking the ability and checking its accuracy before or during battle.
7) Unbending not triggered by Take the Hit
Self damages from Take the Hit does not proc Unbending regeneration. It could have been a nice combo, but nay...
8 ) Garrote complete description
The description is vague, so here is how it actually works :
6s paralysis (modified by INT / PL) as described.
During paralysis, the target takes about 4 raw damages per tick (1s tick !), modified by MIG / PL.
After paralysis, the target is prone (so the 3s Prone are added to Paralysis duration for the total duration of incapacitation)
After paralysis, the target also received 20 raw damages, modified by MIG / PL.
Overall, it sounds nice and scales quite well. On a Single Class Paladin with max MIG and INT, it does around 120 raw damages total. It might be nice with an Helwalker or a SC class Paladin (since DoT gets double benefit from PL and SC Paladin might get a lot of zeal to spend from Divine Retribution and downed summons).